Semiconductor devices including junction field effect transistors (JFETs) have been known. Such a semiconductor device includes a p-type semiconductor substrate, an n-type semiconductor region (semiconductor layer) formed on the p-type semiconductor substrate, an n-type drain region formed in the surface layer of the n-type semiconductor region, a plurality of n-type source regions formed at the surface layer of the semiconductor region with a gap between the n-type drain region and the n-type source regions, a p-type gate region formed at the semiconductor region between the source regions, and a resistive field plate which is disposed on the semiconductor region between the drain region and the source regions, is electrically connected to the drain region and the ground, and has a spiral shape in a top view.
In the above semiconductor device including the resistive field plate, unlike a semiconductor device having a field plate in an electrically floating state, a voltage drop occurs in the resistive field plate. It is ideal that a voltage value at the end of the field plate disposed in the vicinity of the source region is 0V. However, in practice, since there is a voltage drop, the voltage value does not become 0V. Therefore, the source regions and/or the gate region formed at the surface region of the semiconductor layer may be affected by the voltage drop of the field plate formed on the semiconductor layer. Then, the source regions are insufficiently depleted, which may result in decrease in the breakdown voltage of the semiconductor device.
The inventors of the present disclosure thought that the above problem might have been resolved since the effect of the voltage drop of the field plate on the source regions and/or the gate region can be reduced when the field plate is disposed at a position spaced from the source regions. However, in this case, it was found that the breakdown voltage is reduced as a result of concentration of an electric field in an empty region between the source regions and the field plate.